1. Field of the Invention and Definition of Terms
The invention relates to the art of fabricating irregularly shaped preforms of parts from a multiple thickness layup of fiber-reinforced plastic composite material.
In one application of this art, the manufacture of composite helicopter rotor blades, for example, so-called filler segments or parts made from preforms of the kind noted above are utilized. These filler segments or parts typically have irregular shapes to conform them to the shape of corresponding spaces in the rotor blade which are to be filled by the filler segments or parts.
A preform is understood in the art to be an uncured part, i.e., it is a part formed from a layup to a predetermined shape which is yet to be cured into a final structurual part. In the manufacture of helicopter rotor blades the preform may be cured separately, in assembly with the rotor blade, or in any subassembly of the rotor blade, such as the fillers cured with the spar subassembly. For more on the art of fabricating a helicopter rotor blade from a composite material, see U.S. Pat. Nos. 3,237,697 and 4,095,322.
A layup is understood in the art to be an aggregate of individual tape strips (laminates), the tape strips comprising fiber reinforced plastic material. The fibers or filaments are preferably unidirectional for the filler preforms made according to the present invention.
An irregular shaped preform is understood in the art to be one in which at least one edge of the preform transverse cross-section varies along its length with respect to a reference co-ordinate set of axes. For example, a rectangular or square cross-section does not include an edge (any of the four sides) which varies along its length relative to a two-dimensional co-ordinate set of axes, whereas a triangular cross-section does.
This invention employs cold compacting or coining of the layup to form the preform. The cold compacting or coining process involves the application of pressure at essentially room temperature to the layup to deform it to achieve a preform of a desired shape. A tool is used for this purpose and includes a male and female portion, hereinafter referred to as the mold.
2. Description of an Existing Method
The manufacture of irregularly shaped preforms from laminates, rovings and other forms of fiber reinforced resin matrix material has involved, among other steps, producing a layup of many thicknesses of the material in its uncured state formed closely to the desired shape of the preform to be made. Each laminate must be cut by hand using ascissors or a knife edge in progressively developed sizes and shapes so that when laid one on another in a prescribed order the layup closely approximates the desired shape of the preform. A layup conforming nearly exactly to the final shape is inserted into a mold tool whose cavity also closely approximates the outer contour of the preform and of the layup. The mold is closed by a closure mold portion having the desired outer contour formed on its inner surface. Thus, the uncured layup is fully enclosed within the mold and fits the mold cavity with substantial precision. The layup is then cold compacted or coined by the application of pressure to the layup in the mold. The applied pressure supplies the force needed to produce the adherence of the laminates so that the preform will retain its shape prior to being cured. The shape itself, however, is imparted by the noted cutting and hand assembly of the layup.
Since the layup must be produced by hand, the time factor involved is quite high. The fabrication is therefore costly.
Also, in the process of forming the layup, considerable care is taken to force air from between the lamina so that the layup will fit within the mold cavity. To this end, frequent rolling of the tacky lamina after each ply is laid on the preceding ply is desirable in order to expel the air, because if an excessive amount of air remains with the layup its volume may exceed the capacity of the mold to accept its bulk. Even though the mold is sized to accommodate some degree of air entrapment, if this is overdone, the mold may be too large compared to the reduced size the composite attains during the cold compaction or coining process.
The air also presents a problem during the subsequent cure cycle of the preform with the composite blade. The presence of entrapped air in unacceptably large quantities within the layup will produce a cured part whose resin matrix is porous, frothy and aerated in appearance and in fact. Expelling the air, therefore, also contributes to the high time factor.